The present invention relates to a method of polishing a wafer surface in a wiring process as one of processes for manufacturing a semiconductor integrated circuit, and particularly to a method of polishing a thin film layer to be polished on a wafer surface by accurately detecting the thickness of the thin film layer and feedback-controlling the polishing condition on the basis of the detected result.
A wiring process, one of a number of processes for manufacturing a semiconductor device, includes a process of planarizing a micro-topography on the surface of an insulating layer formed on a wafer surface by chemical-mechanical polishing. First, the planarization process will be described in detail with reference to FIGS. 1(a) to 1(f).
FIG. 1(a) shows a sectional view of a wafer on which a metal layer is formed as a first layer. An insulating film layer 2 is formed on the surface of a wafer substrate 1, and a metal layer 3 made of aluminum or the like is provided on the insulating film layer 2. A contact hole 2' is formed in the insulating layer 2 for connecting the metal layer 3 to a transistor portion, and a pit 3' is formed in the portion of the metal layer 3 corresponding to the contact hole 2'. In the next wiring process of forming a second layer, as shown in FIG. 1(B), an insulating layer 4 is formed on the metal layer 3 as the first layer, and an aluminum layer as the second layer is formed on the insulating layer 4. At this time, if being left as deposited, the insulating film layer 4 causes an inconvenience such as defocus upon exposure in the subsequent lithography process because of the micro-topography on its surface. To cope with this inconvenience, the insulating film layer 4 is polished by a manner described later up to a level shown by the dashed line 5, thus planarizing the surface of the insulating film layer 4 as shown in FIG. 1(c). After the surface of the insulating film layer 4 is thus planarized, a contact hole 6 is formed as shown in FIG. 1(d), and a wiring pattern 7 as the second layer is formed thereon as shown in FIG. 1(e). As shown in FIG. 1(f), an insulating layer 8 is then formed again, and polished up to a level shown by the dashed line 9. A multi-layer wiring is thus formed by repeating these steps.
FIG. 2 shows a polishing method for planarizing the above-described insulating film layer. A polishing pad 11 is stuck on a platen 12 and is rotated by a motor 10. On the other hand, a wafer 1 to be processed is fixed on a wafer holder 14 by way of an elastic backing pad 13. The wafer 1 is pressed on the surface of the polishing pad 11 while the wafer holder 14 is rotated. At this time, slurry 15 is supplied onto the polishing pad 11. Thus the projecting portions of the insulating layer on the surface of the wafer 1 are polished off, that is, the surface of the insulating film layer is planarized. In this case, by the use of colloidal silica suspended in a solution of potassium hydroxide as the slurry, there can be obtained a high polishing efficiency being several times or more that in the case where only a mechanical polishing action is imparted because a chemical polishing action is added to the mechanical polishing action. This process has been extensively known as a chemical-mechanical polishing method.
In the above polishing process, a problem lies in how the progress of the polishing up to a level 5 or 8 is detected, and in when the polishing should be completed, that is, in the so-called endpoint detection. Specifically, in the above polishing method, as shown in FIG. 3, the wafer 1 to be processed is put between the two elastic pads 11, 13, and accordingly, it is almost impossible to detect a change in thickness of the insulating film layer in the target level of 0.1 .mu.m by measuring a change in the distance between these pads.
As the prior art endpoint detection technique, there has been used a method of previously examining a polishing rate and estimating a residual thickness by time control; or a method of estimating the progress of polishing by detecting a change in the rotational torque of a rotating platen on the basis of a phenomenon in which a friction force between a polishing pad and a workpiece is changed as the topography on the surface to be processed is reduced along with the progress of polishing (see the Specification of U.S. Pat. No. 5,069,002). Either of these methods, however, has a disadvantage that the detection accuracy is dependent on a change in the polishing condition.
Another prior art is disclosed in U.S. Pat. No. 5,081,421, which takes into account the fact that the insulating film layer to be processed is made of dielectric material and utilizes a phenomenon in which the capacitance of an insulating film layer is changed along with the progress of polishing. Specifically, as shown in FIG. 4, a portion 17 of a conductive metal made rotating platen 12 is insulated from the other members by means of an insulating ring 16, and an AC voltage of about 5 KHz is applied between the portion 17 and a rotating holder 14 for a wafer. In the case of where a wafer substrate 1 and a polishing pad 11 permeated with slurry are conductive, an AC current flows therebetween, and in this case, the current value is dependent on the thickness of the insulating film layer 4 to be polished. Consequently, on the basis of such a change in the current value, the thickness of the insulating film layer 4 can be detected. Even in this case, however, a change in the capacitance along with the progress of polishing is influenced not only by a change in the thickness of the insulating film layer 4 but also by the texture and density of an aluminum wiring 3 as the bottom layer, so that the detection sensitivity must be calibrated for each circuit pattern on the wafer 1.
As a process of polishing the surface of a semiconductor device to which the present invention is applied, there has been known a method of previously forming a metal thin film layer for wiring and then planarizing only projecting portions of the thin film layer. In this case, the above-described method of measuring the film thickness using a change in capacitance cannot be applied. As a method applied to this case, an impedance measurement method utilizing the conductivity of the above metal thin film layer portion is disclosed in EP-A1-0460384; however, this method is disadvantageous in that it cannot be applied to the case of polishing an insulating thin film layer.